<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button class="btn">test</button>
    <script>
        let btn = document.querySelector('.btn');
        btn.addEventListener('click', function () {
            alert('Button clicked');
        });
        let clickEvent = new Event('click');
        btn.dispatchEvent(clickEvent);
    </script>
</body>

</html>